1. Field of the Invention
The present invention relates to a current source. More particularly, the present invention relates to an interconnecting process and a voltage-controlled current source.
2. Description of the Related Art
In some electronic circuits, a voltage-controlled current source is often required to convert voltage signals into current signals. FIG. 1A is a circuit diagram of a conventional negatively related voltage-controlled current source. As shown in FIG. 1A, an input control voltage VBL fed to a transistor P1 is converted to another voltage at VL through a transistor P1 serving as a voltage converter. Then, the voltage difference between the node VL and the DC voltage source VDD is converted to a current flow through a resistor R. Thereafter, through a current mirror formed by a pair of transistors N21 and N22, a current IL is output. When the control voltage VBL increases, the voltage at the node VL will increase. As a result, the terminal voltage difference of the resistor will reduce, leading to a reduced output current IL. Hence, there is a negative relation in this voltage-controlled current source circuit. Because the resistor R receives the direct current voltage source VDD directly, any fluctuation in the DC voltage source VDD will affect the terminal voltage difference of the resistor R and then the voltage of the output current IL. FIG. 1B is a graph showing a relationship between the control voltage VBL versus output current IL for the circuit in FIG. 1A when the voltage of the voltage source VDD fluctuates. As shown in FIG. 1B, the horizontal axis indicates the change in the control voltage VBL and the vertical axis indicates the voltage of current flowing through the transistor N21 (or the voltage of the output current IL). Under the same input control voltage VBL, any changes in the DC voltage source VDD can affect the terminal voltage difference of the resistor R and result in a change in the voltage of the output current IL.
The control voltage (the voltage at the node VL), the real determinant of the voltage of the current flowing through the transistor N21 (or the voltage of the output current IL) still differs from the input control voltage VBL by a gate-source voltage (VGS) of the P-type transistor P1. However, the gate-source voltage (VGS) is not a fixed voltage. In general, the gate-source voltage (VGS) is related to the threshold voltage (Vth) and the output current of the MOS transistor P1. FIG. 1C is a graph showing a relationship between the control voltage VBL versus the output current IL for the circuit in FIG. 1A when the MOS transistor has different threshold voltage. As shown in FIG. 1C, the horizontal axis indicates the change in the control voltage VBL and the vertical axis indicates the voltage of the current flowing through the transistor N21 (or the output current IL). From FIG. 1C, it can be seen that when the input control voltage VBL remains unchanged, any change in the manufacturing process will affect the threshold voltage (Vth) of the MOS transistor P1 and thus the current generated will be different.
FIG. 2A is a circuit diagram of a conventional positively related voltage-controlled current source. As shown in FIG. 2A, an input control voltage VBH fed to an N-type transistor Ni is converted to another voltage at VH through the transistor Ni serving as a voltage converter. Then, the voltage difference between the node VH and a ground is converted to a current flow through a resistor R. Thereafter, through a current mirror formed by a pair of P-type transistors P21 and P22, a current IH is output. When the control voltage VBH increases, the voltage at the node VH will increase. As a result, the terminal voltage difference of the resistor R will increase, leading to an increased output current IH. Hence, in this voltage-controlled current source circuit, there is a positive relation between the control voltage and current source.
FIG. 2B is a graph showing a relationship between the control voltage VBH versus output current IH for the circuit in FIG. 2A when the DC voltage source VDD fluctuates. The horizontal axis indicates the change in the control voltage VBH and the vertical axis indicates the voltage of current flowing through the transistor P21 (or the value of the output current IH). As shown in FIG. 2B, the positively related voltage-controlled current source is hardly affected by any variation in the DC voltage source. This is because the transistor NI isolates the resistor from the DC voltage source VDD.
However, the control voltage (the voltage at the node VL), the real determinant of the voltage of current flowing through the transistor P21 (or the voltage of the output current IH) still differs from the input control voltage VBH by a gate-source voltage (VGS) of the transistor Ni. However, the gate-source voltage (VGS) is not a fixed voltage. FIG. 2C is a graph showing a relationship between the control voltage VBH and the output current IH for the circuit in FIG. 2A when the MOS transistor has different threshold voltage. As shown in FIG. 2C, the output current will change according to any drift in the manufacturing process, resulting in a change in the threshold voltage.